The invention relates to a method and apparatus for directionally drilling a borehole in subsurface earth formations.
During the course of drilling operations it is frequently necessary to change the direction of drilling. By use of directional drilling techniques changing the direction of drilling is usually achieved by drilling a curved borehole section until the borehole is at the desired course, whereupon drilling in a straight direction is continued. Numerous attempts have already been made to provide directional drilling methods whereby the course of drilling may be changed without interrupting drilling. U.S. Pat. No. 2,919,897 describes a drilling assembly comprising a stabilizer that can be brought from a concentric to an excentric position relative to the drill string. The stabilizer can be maintained either in the eccentric or in the concentric position thereof in a fixed orientation in the borehole so that curved and straight borehole sections can be drilled at will.
U.S. Pat. No. 3,667,556 describes a downhole drilling motor of which the output shaft is supported by a bearing which is attached in a pivotable manner to the motor housing. During drilling the housing is kept stationary in the borehole and by pivoting the bearing the bit can be put in a tilted position in the borehole so that the direction of drilling may be varied continuously without interrupting drilling operations.